1. Field of the Invention
The present invention relates to a method of manufacturing a gear, which method is utilized for manufacturing a gear such as a spur gear, a helical gear and a bevel gear, etc. More particularly, the invention relates to a method of manufacturing a gear with a central through hole, which method is suitable for manufacturing a gear having a central through hole by means of improved hot die forging with high efficiency, high precision and elongated die life.
The present invention further relates to a forging die for manufacturing a gear from a hollow material having a central through hole, by means of the improved hot die forging.
2. Description of the Related Art
A method of manufacturing a gear such as a spur gear, a helical gear, and a bevel gear etc., by means of machining has been utilized conventionally in many cases, as described in, e.g., "Mechanical Engineers' Handbook" published by The Japan Society of Mechanical Engineering in May 15, 1988, pages B2-136 to B2-137, section 5.2.7 "Gear cutting and Gear shaving". Such a method employing machining, though, has a drawback that a manufacturing efficiency and a material yield in the method are both low. Therefore, in recent years, a method of manufacturing a gear by means of forging utilizing plastic deformation has been often implemented, as described in the aforementioned "Mechanical Engineers' Handbook" pages B2-100 to B2-106, section 4.2.4 "Forging".
FIG. 2 shows an exemplary process having successive steps (a) to (f) for manufacturing a bevel gear by means of hot forging and trimming. That is, with reference to the process, in first step (a) a solid material 21 for the gear is prepared, and in second step (b) the material 21 is heated to a temperature of, e.g., 1100.degree. C. in a high-frequency heating furnace. In third step (c) the material 21 is deformed to a bevel gear-like forged body 23 by hot forging, and in fourth step (d) foil-like flashes 23b formed at ends of teeth 23a of the forged body 23 are removed by a trimming press. In fifth step (e) shotblast cleaning and bonderizing (zink phosphate coating) is implemented to the forged body 23, and finally in sixth step (f) sizing is implemented to the forged body 23 by a sizing press.
The process employing hot forging and trimming requires carrying out the trimming by a trimming press after the hot forging for removing the foil-like flashes 23b formed in the hot forging, so that the process has a disadvantage in that the process lowers manufacturing efficiency by increasing the number of working steps due to the additional trimming step. Thus, there has been a case wherein full enclosed die forging is employed for manufacturing a gear, as described in the aforementioned "Mechanical Engineers' Handbook" page B2-106, paragraph vii "Full enclosed die forging".
FIG. 3 shows an exemplary process having successive steps (a) to (e) for manufacturing a bevel gear by means of hot full-enclosed die forging, which process is described in "Komatsu Engineering Report, Vol. 32, No. 1" published by Komatsu Seisakusho Co., Ltd. in 1986, page 34. That is, with reference to the process in FIG. 3, in first step (a) a solid material 31 for the gear is prepared, and in second step (b) the material 31 is heated in a high-frequency heating furnace 32 and then cut to a proper length. In third step (c) a whole surface of the material 31 is coated with lubricant 33, and in fourth step (d) the material 31 is heated again in a high-frequency heating furnace 34. Finally in fifth step (e) the material 31 is deformed to a bevel gear-like forged body 35 by hot full enclosed die forging.
In both of the processes in FIG. 2 and FIG. 3, the material 21, 31 in solid shape is deformed to the bevel gear-like forged body 23, 35 by the forging, so that both of these processes have a disadvantage in that each process increases the number of working steps due to additional machining steps for forming a central through hole. Therefore, there has been a case wherein a hollow material is utilized for manufacturing a gear by forging.
FIG. 4 shows an exemplary process having successive steps (a) to (f) for manufacturing a bevel gear with a central through hole by means of hot full-enclosed die forging, which process is described in "Journal of The Iron and Steel Institute of Japan, Vol. 78, August 1992, No. 8" published by The Iron and Steel Institute of Japan in 1992, pages 110 to 116.
That is, with reference to the process in FIG. 4, in first step (a) a solid material 41 for the gear is prepared, and in second step (b) a hollow material 42 is formed from the solid material 41 by machining. In third step (c) a carburized layer is formed on inner and outer surfaces of the material 42 by carburizing, and in fourth step (d) bonderizing and graphitizing are implemented to the surfaces of the material 42 for lubricant coating, then the material 42 is heated to a temperature of 1000.degree. C. and deformed to a hollow gear-like forged body 43 by full-enclosed die forging using a die having a mandrel for the hollow part, further in this step (d) quenching and tempering are implemented to the forged body 43. In fifth step (e) sandblast cleaning is implemented to the forged body 43 for removing scales, and finally in sixth step (f) a finished body (bevel gear) 44 is formed from the forged body 43 by machining.
In both of the hot forging and trimming process in FIG. 2 and the full-enclosed die forging process in FIG. 3, since the solid material 21, 31 is forged for forming only the gear teeth, after the forging, a central through hole is formed to the gear-like forged body 23, 35 by machining, then a back surface of the forged body 23, 35 is finished by machining, and after that, carburizing, quenching and tempering is implemented to the finished body. Consequently, each of the processes has a disadvantage in that each process is unfavorable for improving manufacturing efficiency and each process increases production cost because the process increases the number of working steps due to the machining steps and heat treating steps added to the hot forging step for the gear teeth. Further, each of the processes has a disadvantage in that each process suffers from a lowering of size accuracy as the central through hole is being formed.
On the other hand, in the hot full enclosed die forging process in FIG. 4, contacting time of the forging die to the heated material 42 becomes so long that the forging die is tempered by heat influence from the material 42. Therefore, the forging die having low hardness due to the tempering has a tendency to be unfavorably deformed, so that the die life is shortened, causing increasing production costs.
Thus, it has been proposed to manufacture a high precision gear with a central through hole, with high efficiency and without increasing production costs.